1. Field of the Invention
The present invention relates to an apparatus for detecting a position of a reference pattern or mark formed in a substrate to be aligned with a photomask pattern. More particularly, the present invention relates to a pattern position detection apparatus which can be suitably adapted in a wafer alignment apparatus; in a semiconductor device manufacturing apparatus such as an optical exposure apparatus, an X-ray exposure apparatus, an electron beam exposure apparatus, etc. and or a scanning electron microscope.
2. Description of the Prior Art
Among various types of semiconductor manufacturing apparatuses, an exposure apparatus (aligner or stepper) for transferring a pattern of a photomask or a reticle to a semiconductor wafer must be able to perform correct alignment of the photomask with the wafer. For this type of alignment, a small wafer mark is generally formed in the wafer to be aligned, and a mask mark to be aligned with the wafer mark is also formed in the photomask. The exposure apparatus observes the mask and wafer marks from the side of the photomask through a microscope, photoelectrically detects images of the two marks, and automatically corrects a relative positional deviation between the two marks. With the apparatus of this configuration, the wafer mark is a single linear pattern, which is photoelectrically scanned in a direction to cross the two side edges of the mark. The central position between the side edges is detected using an image signal corresponding to the mark, and the central position is defined as the mark position.
An example of such a method is disclosed in U.S. Pat. No. 4,115,762, wherein symmetrical pattern matching is performed using an image signal. This method is based on the principle that when an image signal is returned from the central point between two edges of a mark, optimal matching can be achieved. In general, a photoresist for exposure purposes is coated on the wafer surface to a thickness of about 0.5 to 2 .mu.m. However, the photoresist coating has an irregular thickness near the mark which forms an uneven pattern. For this reason, when the mark is illuminated with monochromatic light during observation through the microscope, a plurality of parallel interference fringes are formed near the edges of the mark. If the interference fringes are symmetrical at the two sides of the mark, the mark position can be detected with a satisfactory precision by conventional symmetrical pattern matching. However, when a photoresist is coated on the wafer surface by a spin coater or the like, due to the centrifugal force, the photoresist thickness may not symmetrically change at the edges (stepped portions) of the mark and non-symmetrical interference fringes may be formed. This degrades the symmetry of the overall image signal and also means that processing depending on symmetry only may result in mark position detection of low precision.